I tried out the PAIA 2720-3B VC Bandpass Filter on the breadboard. I wasn't initially that impressed with its range. However I swapped the diode for a red led and drove it from an exponential current source, the result was a great increase in the range of the filter. I also changed the way in which the Q control was implemented, resulting in, in my opinion a more natural control contour.

I also made a second version with a simpler current source using just one NPN transistor. The results were still very impressive.

The most interesting characteristic of these filters is the way they behave when overdriven. With high Q settings, the LED becomes modulated by the very signal that it is controlling and things freak out somewhat, the LED flickers too in response to the large signal swing!

I tried out the PAiA 2720-3L Lowpass filter. Again it worked better with LEDs instead of diodes and a current source to drive the LEDs. I chained two Lowpass filter sections together to make a 4 pole filter. It was quite good but would really benefit from having a Q / resonance control. I'm going to have to figure out if this can be done.

I was just trying out the circuit from my last post again and it would seem that 68K is a better choice for R12 as it allows the filter to be completely closed.
I suspect that there might need to be a bit of experimentation with some of the resistor values to accommodate different LED's. Specifically, R9 & R12.Last edited by gasboss775 on Sun May 21, 2017 7:15 am; edited 1 time in total

I have experienced some problems with instability ( it bursts into oscillation ) that seem to be somewhat related to circuit layout ( I've only ever built it on a breadboard )
I did find that putting a 100n ( 0.1uF ) capacitor between the collector or Q2 and ground helped a bit. Does anyone else have any suggestions about making this behave. I was thinking of using either an FET optocoupler or a vactrol instead of the diode to tune the filter in order to avoid the circuit using the same ground as the control signal.

Well for the twin T filter (2720-3b) I'd try with a photocell instead of the diode and a circuit to control the LED. Or a real vactrol instead of a DIY one. But DIY you can use a more adequate range for the photocell and have less work fine-tuning it's right resistance range.

Well for the twin T filter (2720-3b) I'd try with a photocell instead of the diode and a circuit to control the LED. Or a real vactrol instead of a DIY one. But DIY you can use a more adequate range for the photocell and have less work fine-tuning it's right resistance range.

I have several vactrols and have been meaning to try them out in this circuit. Using a diode or LED have their own charms due to the distortion generated when the filter is driven with a strong signal. I do like vactrol based filters though, they have a dynamic character of their own.Last edited by gasboss775 on Mon Sep 17, 2018 3:15 pm; edited 1 time in total

The most interesting characteristic of these filters is the way they behave when overdriven. With high Q settings, the LED becomes modulated by the very signal that it is controlling and things freak out somewhat, the LED flickers too in response to the large signal swing!

The original is included for comparison.

Gasboss, what power are you using for V++ and V--? +/- 15V?_________________please visit my AUDIODIWHY blog and website

The most interesting characteristic of these filters is the way they behave when overdriven. With high Q settings, the LED becomes modulated by the very signal that it is controlling and things freak out somewhat, the LED flickers too in response to the large signal swing!

The original is included for comparison.

Gasboss,j what power are you using for V++ and V--? +/- 15V?

The circuit works from a single +Ve supply rail +12 to + 15 V, 15 would provide more dynamic range for the signal.

Here's some PCB layout for this ,if that's ok . I didn't made the add-on mixer ,just the last linear response version ,wanna try this version , have built this filter once earlier as the original on 18V ,PCB is 70x100mm

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